The coating of substrates, for example, glass faces, by means of cathode sputtering in plasma processes, both in a reactive manner and in a conventional manner, is, for example, known from architectural glass coating. To that end, a plasma which carries material away from a target which is deposited on the substrate, for example, the glass pane, is produced with a current or voltage source. Before the deposit, the atoms can still become connected to gas atoms or molecules in a reactive process in accordance with the desired coating.
In particular in reactive processes, mid-frequency generators (MF generators) which generally operate at a frequency of from 10 to 500 kHz are often used. There are also known pulsed generators, in particular bipolar-pulsed generators. All those generators are included with the term power generator with a periodically changing output signal of the power generator for power supply. The output voltage of those generators is often directed with two electrodes in a plasma process chamber which alternately operate as a cathode and anode and which are both connected to a respective target. There are so-called free-running generators or generators which operate with a controlled frequency. In particular in reactive processes, flashovers which are often extinguished by themselves with the next voltage reversal or at least after a few periods, so-called micro-arcs, are also produced in those generators. However, more energetic and longer-lasting flashovers, so-called arcs, may also occur. It is important to identify such arcs safely, reliably and rapidly. Often, arcs are identified by checking the output voltage for a voltage breakdown or by checking the output current for a voltage increase. Alternatively, an arc may be identified by the difference between the currents to the individual electrodes. A limit value for identifying arcs may be adjusted by the operator. A problem is that, for a periodically changing output signal of the power generator, it is not possible to establish a fixed value for a voltage breakdown or current increase because the current and voltage are intended to change continuously as a matter of principle. In the past, different methods have been developed for identification. In one method, the effective value of the current and voltage is established. Since such an establishment has to be carried out over a plurality of periods, this type of identification of an arc is generally substantially too slow and is often in the region of a few hundred milliseconds.
When the generators are used in the semiconductor production process, in particular flat panel display (FPD) production, however, increased requirements are placed on the generators. Here, arcs are intended to be identified within a few microseconds or even less than one microsecond.
EP1801946 A1 discloses a method for identifying arcs in which there are determined time ranges within which an evaluation signal exceeds or falls below reference values. That step is repeated in a subsequent half-wave of the same polarity. An arc is identified if corresponding time ranges differ by more than a predetermined tolerance. In this method, a plurality of reference values have to be provided for reliable identification of arcs and a plurality of tolerances have to be set for each reference value, which is complex. This method may also result in erroneously identified arcs in the case of a very high number of reference values and very well-adjusted tolerances.